Vice President Joe Biden takes the oath of office during the official swearing-in ceremony at the Naval Observatory on Jan. 20 in Washington. The oath is administered by Supreme Court Justice Sonia Sotomayor as Dr. Jill Biden, right, looks on. (Saul Loeb / The Associated Press, pool)

WASHINGTON — Vice President Biden was formally sworn into office this morning, marking the official beginning his second-term in office.

About 120 friends and guests showed up for the private swearing-in ceremony at the vice president's residence on the grounds of the Naval Observatory. Before the swearing-in, Biden, a Catholic, celebrated Mass. Biden was surrounded by his wife, children and grandchildren for the swearing in.

Biden thanked his family and friends for sharing the day with him before heading out to meet up with President Obama to lay a wreath at Arlington National Cemetery.

"I am honored," Biden said.

Obama will be sworn in later today in a private ceremony at the White House. Both the president and vice president will take part in a ceremonial swearing-in on the West steps of the U.S. Capitol.

Biden picked Justice Sonia Sotomayor for the honor. She became the first Hispanic and fourth female judge to administer an oath of office.

"It's an incredible honor to have Associate Justice Sonia Sotomayor swear me in," Biden said. "I believed strongly that she would make a great justice, and it was one of the greatest pleasures of my career to be involved in her selection to the court. From the first time I met her, I was impressed by Justice Sotomayor's commitment to justice and opportunity for all Americans, and she continues to exemplify those values today. Above all, I'm happy for the chance to be sworn in by a friend — and someone I know will continue to do great things."

Three women have previously sworn in presidents and vice presidents: Judge Sarah T. Hughes swore in President Johnson in 1963; Justice Sandra Day O'Connor swore in Vice President Dan Quayle in 1989; and Justice Ruth Bader Ginsburg swore in Vice President Al Gore in 1997, according to the White House.

Biden used his family Bible for today's ceremony, a 5-inch-thick tome featuring a Celtic cross on the cover. It has been in the Biden family since 1893. He used it each time he was sworn in as a senator and when he was sworn in as vice president in 2009. His son Beau used it when he was sworn in as Delaware's attorney general.

Obama is the seventh president or president-elect to have Inauguration Day fall on a Sunday, and is following the path of predecessors who held a private oath-taking on the constitutionally prescribed Jan. 20 date.

Meanwhile, the Obama Campaign apparatus will be hosting a "legacy conference" with supporters and campaign volunteers in Washington on Sunday to discuss the future about the campaign operation they've built. The president's campaign built an impressive online and data operation. On the agenda are conversations about organizing, the budget crisis and gun violence among other issues.